"According to Rome (and VatII) it has never been "prohibited"."
Why, then, was an indult required?
Novus Ordum => Novus Ordo.
Duh. I was thinking "novus ordure" while I was typing.
Why, then, was an indult required?
Because it was left up to the individual diocesan bishops to "grant" the annulments. It's the bishops call. Even though Rome never prohibited them, many US bishops refused to grant the indult.